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Fog, Thick and Varied
Mon Feb 14, 2005
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Hi, I’m Bryan Yeaton for The Weather Notebook. Today, things will get a little foggy.
Fog is made up of little droplets of water, just like in a cloud. But there are different
ways those drops can get involved. At Dew Point, invisible water in the air will become
visible. We can either by cool the air, since cold air holds less moisture, or by adding
more moisture to the air.
In Advection Fog, warm, moist air moves over a colder surface, cooling it to dewpoint,
and the fog appears. Although this can occur over land, it is far more common on the
ocean: the large fog banks that ruin your day at the beach, and provide hazard to ships
at sea. Such a fog contributed to the sinking of the Andrea Doria in 1954.
Upslope fog forms in hilly regions, when an air parcel is pushed uphill and cooled to
its dewpoint by the adiabatic process, where saturated air loses 3.3 degrees for every
thousand feet of elevation gain. In dry air, the temperature drops 5.4 degrees for every
thousand feet (that’s why mountain tops are colder than lower elevations).
Steam Fog, or frontal fog, becomes visible when cold air moves over a warmer body of
water. It is not as thick as Advection fog, but appears as smoke rising from the ocean,
often on a cold winter’s day. In a similar process, radiation fog reaches up its wispy
tendrils from ponds and lakes in the early morning after a still night, when cold air has
pooled in valley bottoms.
To be considered "fog," visibility must be reduced to less than a kilometer. But don’t let
that cloud your thinking: if you can’t see the car ahead of you, it’s foggy out.
The Weather Notebook is supported by Subaru of America. We are produced by the
Mount Washington Observatory.
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